Fast exception processing with multiple cached handlers

ABSTRACT

Fast exception processing is disclosed. In one embodiment, a system includes a splice cache, an exception logic, and an instrumentation mechanism. The splice cache contains one or more lightweight handlers. The exception logic is coupled to the splice cache and determines whether the corresponding lightweight handler for an exception is located in the splice cache. The instrumentation mechanism is coupled to the splice cache. The instrumentation mechanism inserts the lightweight handler into an execution stream.

FIELD

[0001] This invention relates generally to exception processing, and more particularly to fast exception processing in the context of pipelined processors.

BACKGROUND

[0002] The speeds at which processors perform are increasing due to constantly increasing computing needs. There are a number of ways to increase the speed of the processor, such as decreasing cycle time and reducing the average number of cycles required to execute many instructions collectively.

[0003] A well known way of increasing performance in processors and microcontrollers is to overlap the steps of different instructions using a technique called pipelining. To pipeline instructions, the various steps of instruction execution are performed by independent units called pipeline stages. As the number of stages is increased, while keeping the work done by any given instruction constant, the processor is said to be more heavily pipelined. Each instruction progresses from stage to stage, ideally with another instruction progressing in lock step only one stage behind. Thus, there can be as many instructions in execution, as there are pipeline stages. Each stage of execution is designed to perform its work within the processor's basic machine cycle. Thus, although each instruction requires as many cycles to perform as there are pipeline stages, a pipelined processors throughput is one instruction per cycle. In this manner, pipelining reduces the average number of cycles required to execute many instructions collectively, though it does reduce the total amount of time required to execute any single instruction, by permitting the processor to handle more than one instruction at a time. Thus, pipelining is an architectural technique for improving performance over what can be achieved via processor circuit design improvements alone.

[0004] When a processor is executing, exceptions may occur. An exception is a special condition or event that unpredictably changes the normal flow of control in a program. The software that handles an exception is typically called an exception handler or handler. Generally, exceptions are either fatal or non fatal. A fatal exception is an exception which results in abrupt termination of execution. A non fatal exception does not result in abrupt termination of execution.

[0005] When a non-fatal exception occurs, typically the operating system (OS) must be called and the pipeline is flushed. This causes a severe degradation of performance depending on the type of application the processor is engaged in. If the non-fatal exception is occurring frequently, severe degradation of performance occurs.

[0006] One solution has been to hardwire exception processing. This can result in the instruction pointer not changing and as a result no flushing of the pipeline occurs. An example of this solution is Intel®'s 32 bit (iA32) industry standard architecture (ISA). When a translation look aside buffer (TLB) miss occurs, the ISA actually handles the translation cache miss in hardware, by performing “page table walk” in microcode, as it attempts to find the missing translation. A TLB is a table used in a virtual memory system, which lists the physical address page number associated with each virtual address page number. A TLB is used in conjunction with a cache whose tags are based on virtual addresses. The virtual address is presented simultaneously to the TLB and to the cache so that cache access and virtual to physical address translation can proceed in parallel (the translation is done “on the side”). If the requested address is not cached then the physical address is used to locate the data in main memory. The alternative would be to place the translation table between the cache and main memory so that it will only be activated once there was a cache miss. Since the code to handle the TLB miss event is “hardwired”, no instruction fetch is needed to retrieve the instructions needed for the handler. As a result, no pipeline flush is needed since there is no change in the instruction pointer as a result of the TLB miss event. This approach eliminates the performance degradation caused by the pipeline being flushed. This works but has some drawbacks. One drawback to hardwired control is that only a limited number of exceptions may be hardwired. This approach is not entirely satisfactory because processors generally do not perform the same function all the time. For example, having the TLB miss hardwired does not benefit some applications such as one where only numerical calculations are involved. Another drawback to hardwired control is that hardwired control is fixed and therefore inflexible.

[0007] Other architectures such as the reduced instruction computing (RISC) architectures call the operating system more often in the event of an exception. This allows software to determine how to handle the exception but at the cost of performance. The present invention handles exceptions in a faster and more flexible manner.

SUMMARY

[0008] The present invention relates to fast exception processing. In one embodiment, a system includes a splice cache, an exception logic, and an instrumentation mechanism. The splice cache contains one or more lightweight handlers. The exception logic is coupled to the splice cache and determines whether the corresponding lightweight handler for an exception is located in the splice cache. The instrumentation mechanism is coupled to the splice cache. The instrumentation mechanism inserts the lightweight handler into an execution stream. In addition to this embodiment, the invention includes other embodiments, including a method and a splice cache system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 shows a diagram of a system according to an embodiment of the invention;

[0010]FIG. 2 shows a diagram of an instruction cache according to an embodiment of the invention;

[0011]FIG. 3 shows a flowchart of a method according to an embodiment of the invention; and,

[0012]FIG. 4 shows a diagram of splice cache system according to an embodiment of the invention;

[0013]FIG. 5 shows a diagram of a computer in conjunction with which an embodiment of the invention may be practiced.

DETAILED DESCRIPTION

[0014] In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.

[0015] Referring to FIG. 1, a system according to an embodiment of the invention is shown. The system includes a splice cache 101, one or more lightweight handlers 102, an exception logic 103, and an instrumentation mechanism. The splice cache 101 exists as a block of memory containing the plurality of lightweight handlers 102. The splice cache 101 may be of any size. Each lightweight handler is microcode for handling an exception. The exception is any unpredictable event.

[0016] The exception logic 103 is coupled to the splice cache. The exception logic 103 determines whether an exception has a corresponding lightweight handler in the splice cache 101. The lightweight handlers are exception handlers. The exceptions that are covered may be fatal or non-fatal. In one implementation, only non-fatal exceptions will be handled by the lightweight handler. In other implementations, any exceptions can be handled by the lightweight handler. If the exception is fatal, execution of that program stops. If the exception is one covered by a lightweight handler, then the appropriate lightweight handler in the splice cache handles the exception. Generally, this results in avoiding a full pipeline flush.

[0017] When a lightweight handler from the splice cache 101 handles an exception, the pipeline 105 is able to continue normal operation without flushing the pipeline. In one implementation, the pipeline is stalled and the lightweight handler inserted into the pipeline. The stall is much less performance costly than fully flushing the pipeline. For a frequently occurring exception, this can increase performance significantly.

[0018] The instrumentation mechanism 106 inserts the lightweight handler into an execution stream. The execution stream may be a pipeline or execution stream pipeline.

[0019] One way the lightweight handler may be inserted is to stall the processor and insert the instructions for the lightweight handler into the pipeline. Selection logic such as a multiplexor can be used to select what is coming down from a previous stage or the splice cache. This may be the implementation for an in-order machine. An in-order machine is a process that executes instructions sequentially, that is, in the programmer-specified order. Another way for injecting handlers may be used for out-of-order machines. In an “out-of-order” machine, the instructions are not executed sequentially. An “out-of-order” machine is also known as a dynamically scheduled machine. The instructions to be executed are put into a pool and the processor executes them without regard to the programmer-specified order. For an out-of-order machine, the instructions for the lightweight handler may be inserted into the pool without stalling or blocking the processor pipeline. Another way for injecting handlers may be used for machines that are between “in-order” and “out-of-order”. These types of machines are known as decoupled. It keeps a queue of instructions that have already been fetched and are waiting to execute. The fetching of instructions operates asynchronously from the executing of instructions. In this type of machine, the instructions may be inserted into the queue or pool without stalling the pipeline.

[0020] Another embodiment of this invention includes a dynamic handler determiner 104. The dynamic handler determiner 104 tracks the occurrence of exceptions. The dynamic handler determiner 104 inserts lightweight handlers into the splice cache 101 based on some dynamic criteria. The dynamic criteria may be to insert lightweight handlers for the most frequently occurring exceptions into the splice cache 101. The dynamic criteria may be based on the most recently occurring exception. The dynamic handler determiner 104 could be implemented by hardware or software. The dynamic handler determiner 104 allows a system to maximize the use of its splice cache 101 so that the most frequently occurring non-fatal exceptions are handled by the plurality of lightweight handlers 102 in the splice cache 101. This permits a processor to achieve a high level of performance for various uses. The dynamic handler determiner may insert handlers or have the instrumentation mechanism insert the handlers.

[0021] The lightweight handlers 102 may also be determined by a user. The user can determine which exceptions she would like handled by the lightweight handlers 102. This allows the users to determine how to balance the resources according to their needs.

[0022] The lightweight handlers 102 may also be determined by an application. One application could encounter a large number of floating point exceptions and determine that a lightweight handler for it should be placed in the splice cache 101. Another application may rarely encounter a floating point exception so it would not place a lightweight handler for it in the splice cache 101. Applications may be controlled by a user.

[0023] In another embodiment, a pipeline 105 is coupled to the splice cache 101. The pipeline 105 has earlier stages that must be blocked so that the lightweight handler 102 can be inserted without flushing the pipeline.

[0024] In another embodiment, a pipeline 105 is coupled to the splice cache 101 and the pipeline 105 has a pool so that a lightweight handler may be added to the pool without flushing the pipeline 105. The pool is an instruction pool used in “out-of-order” machines and other types of machines.

[0025] In another embodiment, the splice cache 101 is a separate cache. In another, as shown in FIG. 2, the splice cache 101 is located within an instruction cache 201. The instruction cache 201 is used to cache instructions to be executed by a processor. An extra bit could be tacked on to splice cache instructions to signal exception processing. The system can be implemented either way to make the best use of available resources.

[0026] Referring to FIG. 3, a method according to an embodiment of the invention is described. A determination may be made whether the exception is fatal or nonfatal. A fatal exception is one that will cause a current program to halt. A non-fatal exception will not cause a program to halt. A check is done to see if the exception is covered by any of the lightweight handlers in a splice cache 301. If it is not, typically, a call to the operating system is made to handle the exception and the pipeline is flushed. If the exception is covered by a lightweight handler in the splice cache, the appropriate lightweight handler will process the exception 302.

[0027] In another embodiment, the occurrence of exceptions are tracked. When the occurrence of a exception exceeds a predetermined amount, a lightweight handler for it may be added to the splice cache. Then, a less frequently or less recently used lightweight handler would be removed from the splice cache. This embodiment allows the dynamic alteration of the contents of the splice cache so as to allow the best use of the splice cache.

[0028] In other embodiments, a user, an application, or a processor may determine which lightweight handlers occupy the splice cache.

[0029] Referring to FIG. 4, a splice cache system according to an embodiment of the invention is disclosed. The splice cache system includes a storage structure 401, one or more lightweight handlers 402, and a logic unit 403. The storage structure 401 can be made out of any type of memory or storage medium. For example, the storage structure could be an allocation of virtual memory or space on a hard drive. The storage structure 401 may exist in its own space or it may exist as part of a larger memory.

[0030] The lightweight handlers 402 are located withing the memory space 401. Each lightweight handler is capable of handling an exception. This allows the exception to be handled while avoiding a full pipeline flush.

[0031] The logic unit 403 is coupled to the storage structure. It may be capable of adding and removing lightweight handlers to the storage structure 401. This allows the lightweight handlers used to vary. The logic 403 may be controlled by a user, an application, or a processor. Thus, the plurality of lightweight handlers 402 can vary according to the use or application. The logic unit 403 may also track the occurrence of exceptions and add and remove lightweight handlers according to standard replacement algorithms.

[0032] In another embodiment, a pipeline 404 is coupled to the storage structure 401. The pipeline 404 has a number of stages. The number of stages may vary. A number of stages are blocked or stalled to allow the lightweight handler to enter the pipeline 404. The pipeline 404 may also have a pool and the lightweight handler is then added to the pool instead of blocking stages.

[0033]FIG. 5 is a block diagram of a computer system 500 of an example embodiment of the present invention. The computer system 500 comprises bus 505, keyboard interface 501, external memory 502, mass storage device 503 and processor 504. Bus 505 can be a single bus or a combination of multiple buses. Bus 505 provides communication links between components in the computer system. Keyboard interface 501 can be a dedicated device or can reside in another device such as a bus controller or other controller. Keyboard interface 501 allows coupling of a keyboard to the computer system and transmits signals from a keyboard to the computer system. External memory 502 can comprise a dynamic random access memory (DRAM) device, a static random access memory (SRAM) device, or other memory devices. External memory 502 stores information from mass storage device 503 and processor 504 for use by processor 504. Mass storage device 503 can be a hard disk drive, a floppy disk drive, a CD-ROM device, or a flash memory device. Mass storage device 503 provides information to external memory 502.

[0034] Processor 504 includes a single register file containing both speculative values and architecturally visible values. The processor 504 may be compatible with, but is not limited to, processors such as an Intel® architecture processor, manufactured by Intel Corporation of Santa Clara, Calif., the assignee of the present invention. In alternate embodiments, the processor 504 is compatible with a PowerPC™ architecture processor, an Alpha™ architecture processor, and the like. It is appreciated that at least some embodiments of the processor of the present invention are pipelined.

[0035] The various embodiments may be used or implemented on the computer system described. Other computer systems may also be used.

[0036] The various embodiments described above allow exceptions to be handled in a flexible and fast manner, unlike the prior art. Unlike exception handlers that were hardwired, the exception handling in this invention may be modified while providing the same performance. Unlike exception handling that was previously done by operating systems and software, the exception handling in this invention does not require flushing the pipeline causing a degradation in performance. This allows the instruction stream to continue uninterrupted which will yield higher performance.

[0037] Fast exception processing has been described. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. Therefor, it is manifestly intended that this invention be limited only by the following claims and equivalents thereof. 

I claim:
 1. A system comprising: a splice cache containing one or more lightweight handlers; an exception logic coupled to the splice cache to determine whether the corresponding lightweight handler for an exception is located in the splice cache; and an instrumentation mechanism coupled to the splice cache to insert the lightweight handler into an execution stream.
 2. A system as in claim 1, further comprising: a dynamic handler determiner to track occurrence of exceptions and to insert lightweight handlers based on some dynamic criteria into the splice cache.
 3. A system as in claim 1, wherein the lightweight handlers are determined by a user.
 4. A system as in claim 1, wherein the lightweight handlers is added to the splice cache for the most recently occurring non-fatal exception the least recently used lightweight handler is removed from the splice cache.
 5. A system as in claim 1, wherein the splice cache is separate from an instruction cache.
 6. A system as in claim 1, wherein the splice cache is within an instruction cache.
 7. The system of claim 1, further comprising a pipeline coupled to the splice cache comprising stages wherein earlier stages are blocked so that the lightweight handler may enter the pipeline.
 8. The system of claim 1, further comprising a pipeline coupled to the splice cache comprising a pool so that a lightweight handler may be added to the pool.
 9. A method comprising: checking a splice cache to determine whether an exception is covered by a lightweight handler located in the splice cache; and processing the exception covered by the lightweight handler upon determining that the exception is covered the lightweight handler.
 10. The method of claim 9, further comprising adding a non-fatal exception handler to the splice cache upon determining that the corresponding exception handler is not already in the splice cache and an occurrence of the non-fatal exception handler has exceeded a predetermined threshold such that a less frequently occurring non-fatal exception handler in the splice cache is removed from the splice cache.
 11. The method of claim 9, further comprising loading user identified lightweight handlers to the splice cache.
 12. The method of claim 9, further comprising determining the lightweight handlers located in the splice cache by an application.
 13. The method of claim 9, further comprising dynamically determining the lightweight handlers located in the splice cache by a processor.
 14. A splice cache system comprising: a storage structure; one or more lightweight handlers located within the storage structure such that each lightweight handler corresponds to a exception; and a logic unit coupled to the storage element.
 15. The system of claim 14, wherein the logic unit is able to add and remove lightweight handlers from the storage structure.
 16. The system of claim 15, wherein the logic unit is controlled by a user.
 17. The system of claim 15, wherein the logic unit is controlled by an application.
 18. The system of claim 15, wherein the logic unit is controlled by a processor.
 19. The system of claim 14, wherein the logic unit tracks the occurrence of exceptions and adds the lightweight handler to the storage structure for a most recently occurring exception and removes the lightweight handler from the storage structure for a less recently occurring exception.
 20. The system of claim 14, further comprising a pipeline coupled to the storage structure comprising stages wherein stages are blocked so that the lightweight handler may enter the pipeline.
 21. The system of claim 14, further comprising a pipeline coupled to the storage structure comprising a pool wherein the lightweight handler is placed in the pool. 